United Airlines is to sell up to 30 Boeing 757s to freight operator FedEx Express, with deliveries of the twinjets set to start this year.
Its agreement covers the purchase of 14 initial aircraft by FedEx.
These 757s will be converted to freighters upon delivery, FedEx says in a regulatory filing, following a deal reached on 8 March.
Deliveries will start this year and continue to 2015.
But FedEx says the agreement will also provide for the purchase of up to 16 additional 757s. This extension is "subject to the satisfaction of certain conditions".
FedEx detailed the agreement in a quarterly filing to the end of February 2013.
At the end of 2012 the combined fleet of United Continental Holdings included 154 Boeing 757s, of which 133 were the -200 variant.
United Continental owns 47 of these 757-200s and leases the rest. Its 757 fleet is powered by Rolls-Royce RB211 and Pratt & Whitney PW2000 engines.